The present invention relates to a method for illuminating a spot in a tunnel of a highway, specifically that of illuminating an entrance portion thereof.
In the case of driving a car on a highway, when a driver approaches a tunnel and the tunnel occupies a sufficiently wide area in the driver's range of vision, the driver scans the road surface area from the car to the entrance and judges whether any obstacle exists on the road's surface. Then the driver proceeds while concentrating his line of vision and attention on the dark tunnel. Starting at this point, the center of the driver's line of vision is filled with darkness from inside the tunnel and his visual acuity begins to rapidly decrease. The geographical point at which the driver starts to pay attention to the entrance of the tunnel is called "the beginning attention point". According to an experiment, when the driver does not need to carefully watch any other obstacles on the road such as other cars, he usually pays attention to the entrance of the tunnel during about 80% of the time interval from the afore-mentioned "beginning attention point".
Meanwhile the illumination in the tunnel is weakened gradually in accordance with an increase of distance between the entrance portion of the tunnel and the spot to be illuminated in an inner space thereof. That needs to be changed depending on the open-air brightness condition. Usually in an ordinary tunnel, illumination starts at a point 10 m from the tunnel entrance in connection with the amount of existing natural light and the brightness of the illumination is controlled in proportion to the open-air brightness. However, if the brightness of the illumination apparatus such as a natrium lamp or the like is controlled in a wider range, its average life span may be shortened causing increased maintenance problems and operating costs.